Linet MS, Hatch EE, Kleinerman RA, et al. Residential
exposure to magnetic fields and acute lymphoblastic leukemia
in children. New England J Med 1997;337:l-7.
McBride ML, Gallagher RP, Theriault G, et al.
Power-frequency electric and magnetic fields and risk of
childhood leukemia in Canada. Am J Epidemiology
Green LM, Miller AB, Agnew DA, et al. Childhood leukemia and
personal monitoring of residential exposures to electric and
magnetic fields in Ontario, Canada. Cancer Causes & Control
Day N, et al. Exposure to power-frequency magnetic fields
and the risk of childhood cancer. The Lancet
These four retrospective case/control studies of the
association between childhood cancer and magnetic fields
were begun around 1990 and have been published in the last
several years. They were each intended to employ better
exposure assessment methodology than earlier studies were
felt to have used -- mainly by averaging exposures at
different locations and/or times. But the rationale for
such exposure averaging has never been clearly spelled out
or justified. Briefly, the findings were as follows:
The first study above shows a slight trend in its
results toward a modest risk at higher fields -- a result
too weak to allow interpretation by itself.
The second study, on the other hand, is so strongly
negative in its findings that it actually indicates a
statistically significant protective effect against cancer
for moderately elevated fields (but not significantly
protective for still higher fields). This bizarre finding
casts some doubt on the study's methodology in general.
The third study is the exact reverse, having
indications of higher risk at more moderately elevated
fields than other studies have indicated.
(These two Canadian studies -- having such flamboyant
and such disparate results, have raised questions about the
unusual recording meter employed in both, and the unique
procedure of having case and control children themselves
wear the meter after diagnosis and treatment of the cases.
We have addressed these questions in: "Leeper E. and
Wertheimer N. Potential motion related bias in the worn
dosimeter measurements of two childhood leukemia studies,
The fourth study showed little trend of any kind in its
findings. However, its exposure assessment methodology is
quite different from that of the earlier work (including its
use of averaging), so the failure to find a risk increment
may simply be due to its asking a different question. In
addition, the study population included only a very sparse
number of those with the high-field exposures that have been
implicated by previous studies. But for leukemia those few
high-field exposures were found somewhat more often for
cases than controls (five cases against three controls above
4 milligauss). This dearth of high-field subjects may
reflect a difference In British electrical distribution, but
almost certainly was exacerbated by their use of exposure
averaging (which will tend to "pull down" many of the
already-rare high exposure levels measured). This is the
largest study done to date, though certain short-cuts were
Interpreting the study results -- pooled analyses
So how can a concerned lay person cut through the confusing
and contradictory array of scientific findings?
Scientists believe in what is being done and its ability to
arrive eventually at a consensus understanding, but we
aren't there yet. We like to say, "more work is needed,"
and we apologize if that sounds like we're running a society
for the preservation of jobs for scientists. We are truly
sorry not to be able to give the immediate, firm, confident,
unanimous answers that people would like; but this is a
One approach that has been taken to the uncertainty of the
individual results is to pool the data from various
epidemiologic studies to create one large "meta-analysis."
This approach has several things going for it:
(1) A problem that has plagued interpreting the work in this
area is that for the most part the strongest findings have
occurred at field levels (above 3 or 4 milligauss) seen at
only a small percentage of houses (except if measured at
localized "hot spots" in other houses). By combining
studies, the effective "sample size" of highly exposed
subjects is improved.
(2) An individual study may have certain flaws in its
procedures (usually ones that are unsuspected or are
unavoidable), so it helps to combine different studies that
may at least have different flaws at different places and
perhaps in different directions.
(3) If different studies have used different ways of
evaluating magnetic fields, it may be possible (often with
the cooperation of the original researchers) to extract
those portions of each study's data where the field
evaluation procedures used were similar to those of the
other studies being considered.
It should be said that this kind of meta-analysis is itself
difficult, and comes with its own pitfalls. But it does
provide an important way of summarizing and combining the
work from a number of published studies.
The two pooled analyses listed below attempted to combine
comparable results from various published epidemiologic
studies of childhood cancer and magnetic fields -- using, as
much as possible, similar measurements taken at similar
locations and times, though the individual studies may have
branched out into various unique analyses of their own
(including complex protocols of field averaging for each
Both analyses found, among the various studies, a fairly
consistent indication of tendency toward an elevated
childhood cancer risk at residential field levels above
about 3 or 4 milligauss. In most of the separate studies
there had been an excess of cases (vs. controls) seen at
such levels, but too few subjects in each study at those
levels to draw statistically significant conclusions.
However, for the larger group of subjects in the combined
data, that high-field-level excess of cases was significant.
Greenland S, Sheppard AR, Kaune WT, Poole C, Kelsh MA. A
pooled analysis of magnetic fields, wire codes, and
childhood leukemia. Epidemiology 2000; ll:624-634.
Ahlbohm A, Day N, Feychting M, Roman E, Skiner J, Dockerty
J, Linet M, McBrice M, Michaelis J, Olsen JH, Tynes T,
Verkasallo PK. A pooled analysis of magnetic fields and
childhood leukemia. British J Cancer 2000; 83:692-698.
There have been several reviews of past work in which a
group of scientists expert in the area and holding various
views was convened to evaluate the scientific findings to
date (including addressing the difficult questions of the
quality and merits of the work done). The first listed
below was the largest undertaking. Each of these groups,
with more or less internal unanimity, found that the
evidence indicated magnetic fields have a possible link
to the risk of leukemia In children:
Assessment of Health Effects from
Exposure to Power-Line Frequency Electric and Magnetic Fields. National Institute
of Environmental Health Sciences, NIH Publication No.
committee report to the U.K. National Radiological Protection Board (2001).
the working panel assembled by the International Agency for Research on Cancer (2001).
Over earlier years, a number of other reviews of the
scientific evidence have been published by groups or
individuals -- in some cases putting what can only be
described as a spin on their descriptions.
Note that despite what has been said, there is laboratory
work supporting the possibility of a cancer effect, although
it too has its ambiguities and incompleteness -- and
although one of the prominent (but perhaps abrasive) players
in that area has been successfully pilloried for his pains.
In general, we seem to be muddling through to some kind of
consensus that there is indeed something going on here, even
if we are a long way from understanding exactly how much,
where, and how it happens.